In recent years, ultrasonic transmitter-receivers have been industrially utilized in a wide variety of fields of distance measurement, object detection, flow measurement, robot control and the like.
As a first ultrasonic transmitter-receiver, there is the ultrasonic transmitter-receiver described in Japanese Examined Patent Publication No. 6-101880. Construction and operation of this conventional ultrasonic transmitter-receiver will be described with reference to FIG. 10.
FIG. 10 is a sectional view of a first conventional ultrasonic transmitter-receiver. In FIG. 10, reference numeral 100 denotes an ultrasonic transmitter-receiver, 101 an ultrasonic transducer, 102 an acoustic matching layer, and 103 a housing.
In the construction of FIG. 10, operation during wave transmission will be described first. The ultrasonic transducer 101 receives a drive signal given from a drive circuit (transmitter circuit 701) via signal wires 104 and generally generates ultrasonic vibrations at a frequency in the vicinity of a resonance frequency of the ultrasonic transducer 101. The ultrasonic vibrations generated at the ultrasonic transducer 101 are transmitted to a fluid around the ultrasonic transmitter-receiver 100 via the acoustic matching layer 102. The acoustic matching layer 102 is constructed of a material having acoustic impedance intermediate an acoustic impedance of a circumjacent fluid and an acoustic impedance of the ultrasonic transducer 101, and has a function to improve wave transmission efficiency to the circumjacent fluid.
A piezoelectric ceramic is typically used for the ultrasonic transducer 101 that generates ultrasonic vibrations, and its acoustic impedance is, for example, about 30×106 kg·m−2·s−1. When the circumjacent fluid is a gas of air or the like, the acoustic impedance of, for example, air is about 400 kg·m−2·s−1, the acoustic impedance of the acoustic matching layer 102 is set to about 0.11×106 kg·m−2·s−1, and a thickness is preferably set to a quarter of a wavelength at an estimated ultrasonic frequency.
Conventionally, in order to form a matching layer that has an acoustic impedance intermediate those of the piezoelectric transducer and air, there is used a material obtained by solidifying a material (for example, glass balloons or plastic balloons) of a comparatively small density with resin.
Operation during ultrasonic wave reception will be described next. An ultrasonic wave, which has propagated through the circumjacent fluid and reached the ultrasonic transmitter-receiver 100, is transmitted to the ultrasonic transducer 101 via the acoustic matching layer 102 conversely to ultrasonic wave transmission. The ultrasonic transducer 101 converts a dynamic action of the ultrasonic wave into an electric signal, and the signal is transmitted to an electric processing section (not shown) via the signal wires 104.
During these transmission and reception operations of the ultrasonic transmitter-receiver 100 described above, transmission and reception of an ultrasonic wave are effected in a direction in which the ultrasonic transducer 101 and the acoustic matching layer 102 are laminated, i.e., in a perpendicular direction of the acoustic matching layer 102.
As a second conventional ultrasonic transmitter-receiver, there is, for example, the ultrasonic transmitter-receiver laid open in the ultrasonic flowmeter described in Japanese Unexamined Patent Publication No. 2000-304581. Construction and operation of this conventional ultrasonic transmitter-receiver will be described below with reference to FIG. 11.
FIG. 11 is a sectional view of a second conventional ultrasonic transmitter-receiver. In FIG. 11, 104 denotes a first acoustic matching layer and 105 a second acoustic matching layer. The first acoustic matching layer 104 has a structure in which a plurality of layers of material plates (104a, 104b, 104c, . . . ) differing in density and acoustic velocity are laminated and these materials are laminated in a descending order of magnitude of acoustic velocity.
Operation of the ultrasonic transmitter-receiver 100 in the construction of FIG. 11 will be described below. During wave transmission, an ultrasonic wave generated by the ultrasonic transducer 101 propagates through the first acoustic matching layer 104 (104a, 104b, 104c, . . . ) and enters the second acoustic matching layer 105 by a drive signal applied from signal wires (not shown). A time during which the ultrasonic wave passes through each layer (104a, 104b, 104c, . . . ) of the laminated first acoustic matching layer 104 is set so as to become equalized, and a wave front of the ultrasonic wave coincides at an interface between the first acoustic matching layer 104 and the second acoustic matching layer 105. That is, the wave propagates in a perpendicular direction at an interface to the first acoustic matching layer 104 in the second acoustic matching layer 105.
The ultrasonic wave, which has propagated through the second acoustic matching layer 105, is refracted by a difference in acoustic velocity between the second acoustic matching layer 105 and an interface of a circumjacent fluid, and radiated to the circumjacent fluid with a direction thereof changed.
During wave reception, the ultrasonic wave, which has propagated through the circumjacent fluid and reached the ultrasonic transmitter-receiver 100 through a process reverse to wave transmission, is refracted at the interface to the second acoustic matching layer 105 to enter the second acoustic matching layer 105, and converted into an electric signal by the ultrasonic transducer 101 via the first acoustic matching layer 104. In this case, an acoustic wave arriving from a direction of wave transmission is selectively received.
The second conventional ultrasonic transmitter-receiver, which can integrate the ultrasonic transmitter-receiver with a wall of a measurement channel when being applied to an ultrasonic flowmeter, since the direction of the acoustic wave is changed by utilizing refraction, therefore has an advantage in that no disorder of flow of the fluid to be measured is generated.
However, there has been an issue in that a propagation loss has inevitably occurred and efficiency of wave transmission and reception has been reduced even if a matching layer of a low density, like the first conventional ultrasonic transmitter-receiver, is used when propagating an ultrasonic wave from an ultrasonic transducer of piezoelectric ceramic or the like into a gas of air or the like. A reason why it is difficult to make an ultrasonic wave efficiently propagate from a solid to a gas is that acoustic impedance of the gas is much smaller than acoustic impedance of the solid, and a strong reflection of an ultrasonic wave disadvantageously occurs at an interface even if a matching layer is interposed.
Further, an ultrasonic transmitter-receiver of a type that effects deflection of an ultrasonic wave utilizing refraction exhibited by the second conventional ultrasonic transmitter-receiver has had an issue in that it has not substantially been applicable as a consequence of a significant reduction in wave transmission and reception efficiency when an angle of deflection is increased due to an additionally inflicted loss caused by the angle of deflection.
Accordingly, the present invention is made in view of the aforementioned issues and has an object of providing a highly sensitive ultrasonic sensor that can deflect an ultrasonic wave and has a high efficiency of wave transmission and reception.